Not perpetual fuel

Unless someone has repealed the second law of thermodynamics, there is no prospect whatsoever of producing fuel from air without using more energy than that fuel delivers (27 October, p 20).

But that is not the economic question: you do not reject your laptop or mobile telephone just because the battery requires more energy to charge than it returns.

If fuel from air is anything, it is a form of energy storage, not generation.

• Sorry, this was a case of poor wording. You are right: the point is that the energy used in producing the fuel should not be too high and the ultimate cost of the fuel should not be prohibitive. If renewable energy is used to generate the gasoline, which when burned returns the same carbon dioxide to the air, it is a carbon-neutral form of energy storage.

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